Fluororubber is drawing the attention as a material for fuel hoses, etc., due to its excellent heat resistance, oil resistance, ozone resistance, chemical resistance, and the like, but the high cost of fluororubber presents a problem.
For this reason, a double-layer structure comprising an innermost layer made of fluororubber to maintain oil resistance as well as flexibility of the hose, and an outer layer made of synthetic rubber, is considered optimal. Among various double-layer structures, one using NBR (acrylonitrile butadiene rubber), rubber that offers relatively favorable oil resistance and is also affordable, is considered the best. However, in general, adhering fluororubber and synthetic rubber directly by means of vulcanization has been difficult.
Accordingly, Patent Literature 1, for example, proposes a rubber laminate of fluororubber and NBR characterized in that 0.3 to 5 parts by weight of at least one chemical compound selected from a group that includes 1,8-diazabicyclo (5,4,0) undecene-7 (DBU), 1,5-diazabicyclo (4,3,0) nonene-5 (DBN) and salts thereof, 0.5 to 30 parts by weight of at least one alkali substance selected from a group that includes calcium hydroxide, sodium carbonate, potassium carbonate, sodium meta-silicic acid and calcium oxide, 1 to 7 parts by weight of organic sulfur donor, and 0 to 0.5 parts by weight of sulfur, are contained per 100 parts by weight of NBR. Because DBU, DBN, DBU salt or DBN salt is used, however, scorching tends to occur when the synthetic rubber blend is kneaded, stored and formed into a laminate even when anti-scorching agent, etc., is added with the aim of preventing scorching. For this reason, any fundamental scorching prevention measure requires that DBU, DBN, etc., not be used. Nevertheless, vulcanization adhesion becomes difficult if DBU, DBN, etc., is not used.
Additionally, Patent Literature 2 present vulcanization adhesion of fluororubber and NBR/PVC, while Patent Literature 3 presents vulcanization adhesion of fluororubber and NBR/EPDM, where, in both cases, vulcanization adhesion involves fluororubber and a rubber composition consisting of not only NBR, but also with PVC or EPDM blended and DBU salt compounded further.
Patent Literature 4 describes that by using a fluororubber layer which contains specific fluororubber and specific acrylic rubber and comprises a layer constituted by a composition which permits peroxide co-crosslinking, a laminate offering excellent adhesion with a layer constituted by other rubber can be obtained.
Under vulcanization adhesion described in any one of Patent Literatures 1 to 3 above, however, use of DBU, DBN, etc., makes it necessary to still implement scorching prevention means. Otherwise, the only way to improve adhesion is to make the fluororubber layer from a composition combining limited ingredients in order to improve adhesion property, as described in Patent Literature 4.
There have been calls for specifically ascertaining a vulcanization adhesion formula of fluororubber and synthetic rubber, to provide a vulcanization-adhered laminate that takes advantage of this formula to firmly adhere fluororubber and synthetic rubber by means of vulcanization without adopting any anti-scorching means necessitated by use of DBU, DBN, etc.